Musannaf Ibn Abi Shayba 36:100

Waki’ narrated to us, from ‘Amr ibn ‘Uthman ibn Mawhab, who said: I heard Abu Burdah say: The Messenger of Allah (may Allah bless him and grant him peace) wrote to a man from the People of the Scripture: "Submit (embrace Islam)." He said: The Prophet (may Allah bless him and grant him peace) had not yet finished his letter when a letter came to him from that man, in which he was greeting the Prophet (may Allah bless him and grant him peace) with peace. So the Prophet (may Allah bless him and grant him peace) replied at the bottom of his letter.